A flat film covered battery is known, which includes an outer cover member formed of a metal-synthetic resin laminate film and a battery element having a plurality of positive electrode plates, negative electrode plates and separators alternately stacked together and accommodated together with an electrolytic solution in the outer cover member (see Patent Documents 1 and 2).
This type of film covered battery is configured as, for example, a lithium ion battery. Patent Document 2 discloses that, after the completion of the film covered battery as the lithium ion battery, the film covered battery is inspected for whether a metal terminal led out from between mating faces of the outer cover member is properly insulated from a metal layer of the outer cover member. In Patent Document 2, a first insulation failure inspection is performed with the application of an impulse voltage between the metal terminal and the metal layer of the outer cover member after placing the battery element in the outer cover member and heat-sealing peripheries of the laminate film of the outer cover member except the vicinity of the metal terminal (before filling the electrolytic solution); and a second insulation failure inspection is similarly performed with the application of an impulse voltage between the metal terminal and the metal layer of the outer cover member after filling the electrolytic solution into the outer cover member and completely sealing the outer cover member.
As a matter of course, the electrode of the battery element and the metal layer of the outer cover member have to be insulated from each other during use of the battery. It is thus preferable to inspect the occurrence of an insulation failure between the electrode of the battery element and the metal layer of the outer cover member under as close a condition as possible to the actual use of the battery. As mentioned above, the insulation failure inspection is performed after filling the electrolytic solution into the outer cover member and completely sealing the outer cover member in Patent Document 2. In this inspection, the film covered battery to be inspected is in a free state.
When a plurality of flat film covered batteries are stacked and used as a battery module, it is conceivable to exert some pressure on the film covered batteries in a direction perpendicular to main surfaces of the outer cover members (i.e. in a stacking direction of the positive electrode plates and the like) during use for the purpose of preventing displacement of the film covered batteries. Further, the film covered batteries may receive some pressure during transportation after completion or during battery module assembling. As the outer cover members are each formed of a laminate film and are thus flexible, there is a case that the internal battery elements may be pressurized in the stacking direction by deformation of the outer cover members under such external pressure.
Accordingly, it is not always possible to obtain adequate inspection reliability in the conventional insulation failure inspection where the film covered battery is inspected in the free state.
In view of the foregoing, it is an object of the present invention to provide a method for more reliable insulation failure inspection of a film covered battery.